[box-tree-api] Explain css pseudo elements

[atanassov]

CSS pseudo elements need to be explained and exposed as part of the CSS Houdini efforts. Let's start with moving this work to the Bot Tree API spec.

[SamB]

Well, the plan was to resurrect the PseudoElement interface that got removed in February 2016, as per WG resolution ...

Oh, I guess that's what the CSSPseudoElement Interface in the CSS Pseudo-Elements Module Level 4 is. It's still in the design phase, though.